I've retested using the internal (9v) battery connected to the b40. The
results are the same as when connected to the 12v glider battery
circuit. Namely, the b40 is unaffected by the transmit so long as the
DAD is not connected to it.

I'm endeavoring to obtain an SWR analyzer to measure the radio
antenna's performance. Short of that, is there any other way for me to
tell if there is too much RFI behind the panel or the DAD just has some
problem that is amplifing an acceptable level of RFI?